SIP (Session Initiation Protocol) is a standard defined as one of call control protocols achieving call control such as a connection request with a real-time application of an internet phone based on VoIP (Voice over Internet Protocol). (For example, see Non-Patent Document 1.)
SIP was used originally for VoIP communication between computers such as personal computers and servers connected via a wired network on the Internet. In recent years, however, an SIP server of wireless carriers (operators) and a wireless communication terminal (apparatus) supporting SIP have been developed, us digitalization of and adoption of IP technology to wireless communication equipment has been progressed. Moreover, recent development of wireless communication technology enables a single wireless communication terminal to be provided with a plurality of wireless communication devices connectable to different wireless communication systems (wireless communication networks). Typical applications are PDA, PC and a cellular phone provided with a first wireless communication device connectable to a cellular phone network and a second wireless communication device to connect to a station of a wireless LAN such as WiFi and WiMAX, or MAN (Metropolitan Area Network).
Non-Patent Document 1: RFC3261 (a document of RFC defining SIP)